ReentrantLock is not showing expected result

Question

Where is the problem? ReentrantLock is not showing expected result. Two threads are executing same time rather than waiting one thread.

class MyThread2 extends Thread{
    String name;
    ReentrantLock reentrantLock = new ReentrantLock();
    MyThread2(String name){
        this.name = name;
    }
    public void run(){
        do {
            try {
                if (reentrantLock.tryLock(20,TimeUnit.MILLISECONDS)){
                    System.out.println("executing : "+ name);
                    Thread.sleep(500);
                    reentrantLock.unlock();
                    break;
                }else {
                    System.out.println("waiting "+ name);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }while (true);
    }
}

public class LockDemo2{
    public static void main(String[] args) {
        new MyThread2("Thread - 1").start();
        new MyThread2("Thread - 2").start();
    }
}

output:

executing : Thread - 1
executing : Thread - 2

score 5 · Accepted Answer · answered Apr 28 '18 at 15:43

You should use the same ReentrantLock in different threads, instead creating different locks.

Change the constructor to this :

ReentrantLock reentrantLock;
MyThread2(String name, ReentrantLock lock){
    this.name = name;
    this.reentrantLock = lock;
}

And pass the same lock to them:

ReentrantLock lock = new ReentrantLock();
new MyThread2("Thread - 1", lock).start();
new MyThread2("Thread - 2", lock).start();

score 1 · Answer 2 · answered Apr 28 '18 at 18:36

You are using two different ReentrantLock on two different threads. @user6690200 solution is correct of using the same lock via the constructor.

Another solution is you can change your ReentrantLock to:

private static final ReentrantLock reentrantLock = new ReentrantLock();

So that each instance of a different thread will always share only one object of your ReentrantLock.

ReentrantLock is not showing expected result

2 Answers2